The present invention relates to a power supply and more particularly to a synchronous rectifier circuit and a power supply used in electronic equipment.
A conventional power supply as shown in FIG. 2 is known in which DC electric power inputted from a DC input power supply 60 to an input unit 51 including an input condenser 61 is switched by a switching unit 52 on the basis of a control signal produced by a driving unit 70 to supply electric power to a load 66 from an output unit 53 including a diode 63 and an output filter 55. Further, a voltage or a current supplied to the load 66 is detected by a detection unit 67 and the detected value is compared with a control target value for the load 66 set in a setting unit 68 by a comparison operation unit 69, so that the control signal based on its comparison result is supplied from a driving unit 70 to the switching unit 52.
A definite circuit of the power supply of FIG. 2 is schematically illustrated in FIG. 3. The switching unit 52 is constituted by an active element (for example, transistor, power MOSFET or the like. In the description of this invention, power MOSFET is simply referred to as MOSFET.) 62. The output unit 53 includes a commutation diode 63 and the output filter 55 composed of a choke coil 64 and a condenser 65. A control unit 54 includes the comparison operation unit 69, the setting unit 68 and the driving unit 70. Further, the control unit 54 includes an oscillation circuit not shown and supplies a pulse signal from the driving unit 70 to the active element 62 to thereby switch a DC voltage Vin applied to the active element 62 from the DC input power supply 60.
In the power supply shown in FIG. 3, when the active element 62 is on, the DC electric power is charged in the choke coil 64 and the condenser 65 and supplied to the load 66. When the active element 62 is off, the energy charged in the choke coil 64 and the condenser 65 is supplied to the load 66 through the commutation diode 63.
At this time, in the control unit 54, the comparison operation unit 69 monitors an output voltage Vo detected by the detection unit 67 and compares the detected output voltage Vo with the control target value set in the setting unit 68 to thereby supply the control signal based on the comparison result from the driving unit 70 to the switching unit 52. Thus, the active element 62 is turned on and off to control so that the electric power supplied to the load is equal to the control target value. The output voltage Vo at this time is expressed by the following equation (1):VO=Vin×(Ton/T)  (1)where Vin represents the DC input voltage, T a period of the pulse signal produced by the driving unit 70, and Ton an on time of the active element 62 within the period T. That is, Ton/T represents a duty ratio.
FIG. 5 illustrates another conventional power supply of the synchronous rectification type using an MOSFET on the commutation side. The power supply has a smaller voltage drop as compared with the case of the diode since the current-to-voltage characteristic of the MOSFET is linear depending on a gate voltage thereof.
FIG. 7 schematically illustrates a feedback capacitance Crss and a gate-to-source capacitance Ciss of a commutation MOSFET 3 of the power supply of the synchronous rectification type. Referring now to FIG. 7, the phenomenon of turning on the commutation MOSFET 3 in the off state when a rectification MOSFET 2 is turned on, that is, the so-called “self-turn on” phenomenon is described. When the rectification MOSFET 2 is turned on in the case where the commutation MOSFET 3 is in the off state, the drain voltage of the commutation MOSFET 3 is suddenly changed to the voltage Vin of an input power supply 1 and accordingly the gate-to-source capacitance Ciss is charged through the feedback capacitance Crss, so that the commutation MOSFET 3 which must be in the off state originally is turned on. In other words, when the gate-to-source voltage Vgs of the commutation MOSFET 3 represented by the following equation (2) exceeds a threshold voltage Vth, the self-turn on phenomenon occurs.Vgs=(Crss/Ciss+Crss)×dVds  (2)where dVds represents a changed amount of the drain-to-source voltage of the commutation MOSFET 3.
A semiconductor integrated circuit such as a microprocessor is supposed as the load of the power supply of the synchronous rectification type shown in FIGS. 5 and 7. Recently, there is the tendency that an operating voltage of the semiconductor integrated circuit is reduced, and an output voltage of the power supply is also required to be reduced in response to the reduced operating voltage of the semiconductor integrated circuit. On condition that the voltage of the DC input power supply is fixed, the on time Ton of the rectification MOSFET 2 represented by the equation (1) is made short and the on time of the commutation MOSFET is made long to thereby reduce the output voltage.
MOSFETs used in the switching power supply such as the power supply of the synchronous rectification type are different from ideal switches and produce loss. The loss can be divided into the loss produced in the on state of the MOSFET, that is, the conduction loss and the loss produced when it changes from the off state to the on state or from the on state to the off state, that is, the switching loss.
In the power supply having a low output voltage, the loss of the rectification MOSFET 2 having the short on time is predominantly the switching loss and the loss of the commutation MOSFET 3 having the long on time is predominantly the conduction loss.
The conduction loss is proportional to the on resistance which is a resistance of the MOSFET in the on state thereof and the switching loss is proportional to a feedback capacitance. Accordingly, an MOSFET having the small feedback capacitance is used for the rectification MOSFET 2 in which the switching loss is predominant and an MOSFET having a small on resistance is used for the commutation MOSFET 3 in which the conduction loss is predominant to thereby reduce the total loss.
Further, as shown in FIG. 9, it is heretofore known that a parallel circuit composed of a resistor 21 and a diode 22 is connected to the gate of the rectification MOSFET 2 in order to reduce or shorten the time that the rectification MOSFET 2 is turned on. A gate voltage of the rectification MOSFET 2 rises slowly because of the resistor 21, so that the changed amount dVds of the drain voltage Vds represented by the equation (2) is small and accordingly the self-turn on phenomenon is difficult to occur. On the other hand, the pulling out of electric charges in the gate of the rectification MOSFET 2 upon turning off is made at high speed since the charges pass through the diode 22.
Moreover, as shown in FIG. 10, it is heretofore known that a capacitor 23 and a discharge resistor 24 are connected to the gate of the commutation MOSFET 3. In this prior art, the gate voltage is supplied through the capacitor 23 and accordingly when the electrical potential at the gate terminal 25 is changed from a positive potential to a ground potential, a gate potential 26 of the commutation MOSFET 3 is driven to a negative potential, so that the commutation MOSFET 3 is difficult to occur when the rectification MOSFET 2 is turned on.